Solar cells in which the active layers are composed of low-mobility semiconductors characteristically exhibit a fall-off in internal collection efficiency under strongly absorbed radiation, contrary to the predictions of simple theory (W. W. Gartner, Phys. Rev. 116, 84 (1959)). This effect has been experimentally observed in amorphous (abbreviated a-)Si cells in several configurations, including Schottky barrier devices (D. Gutkowicz-Krusin, C. R. Wronski, and T. Tiedje, Appl. Phys. Letters 38(2), 87 (1981)), MIS devices, and homojunction (p-i-n) devices (A. E. Delahoy and R. W. Griffith, J. Appl. Phys. 52(10), 6337 (1981)). The effect is attributed in all cases to the back-diffusion of photogenerated carriers toward the illuminated front contacting layer of the cell, where they recombine uselessly (i.e. non-productively) with thermally generated carriers of opposite type. Back-diffusion is most pronounced at short wavelengths, because the carrier concentration gradients which drive it are largest immediately adjacent to the front contacting layer, where short wavelength radiation is absorbed. This interpretation has been supported by two theoretical studies, one involving direct analytical solution of the transport equations (J. Reichman, Appl. Phys. Letters 38(4), 251 (1981)) and the other involving computer simulation of transport in an n-i-p structure (G. A. Swartz, J. Appl. Phys. 53(1), 712 (1982)). These theoretical studies have been criticized, however, on the ground that the assumption of complete thermalization of the diffusing carriers may be unwarranted (A. Rothwarf, Appl. Phys. Letters 40(8), 694 (1982)). Nevertheless it seems well established that back-diffusion constitutes a major recombination pathway, and that its elimination or reduction would lead to higher conversion efficiency.
In order to minimize back-diffusion Reichman (J. Reichman, supra) proposed the use of MIS or semiconductor-electrolyte configurations, in which the necessity to tunnel through an intervening barrier region reduces the effective velocity of carriers moving toward the front surface. This remedy fails in the presence of a high density of interface states (H. C. Card and E. S. Yang, Appl. Phys. Letters 29(1), 51 (1976)), and, in any event, is inapplicable to the currently favored n-i-p and p-i-n configurations. In the case of Schottky barrier devices, Reichman proposed to decrease the width of the depletion layer by uniform doping, thereby increasing the drift field opposing back-diffusion. Calculations show a beneficial effect on short wavelength response, but only at the expense of long-wavelength response. Hence this remedy, too, has limited usefulness.
Nevertheless, Reichman's proposal stimulated several experimental investigations of intentionally doped i-layers in n-i-p and p-i-n devices, of which the first appears to be that of Haruki et al. (H. Haruki, H. Sakai, M. Kamiyama, and Y. Uchida, Solar Energy Materials 8, 441 (1983)). Maximum conversion efficiency is attained for boron concentrations of the order (1-3).times.10.sup.17 atoms/cm.sup.3 and is higher for n-i-p than p-i-n devices. The boron concentration profile is essentially uniform throughout the i-layer, except for a narrow (.apprxeq.500 .ANG.) transition region adjacent the p-layer. The beneficial effects of boron doping are attributed by these authors to an increase in the hole mobility-lifetime product, and not to an alteration of the internal field profile. Shortly thereafter Moustakas et al. (T. D. Moustakas, H. P. Maruska, R. Friedman, and M. Hicks, Appl. Phys. Letters 43(4), 368 (1983)) reported an investigation of the effects of uniform boron doping in Schottky barrier and n-i-p devices. Improved short-wavelength response is found when residual phosphorus impurities in the i-layer are slightly overcompensated. In this case, however, beneficial effects are attributed to the redistribution of internal electric field, without mention of the back-diffusion problem. On the basis of these results Moustakas et al. supra suggest the possibility of tailoring the boron concentration, presumably in a nonuniform manner, in order to achieve an optimum field distribution. Quite recently Hack et al. (M. Hack, M. Shur, W. Czubatyj, and J. McGill, IEEE Trans. Electron Devices, May, 1984 (to be published)) have studied, by computer simulation, the effects of "boron profiling" in an n-i-p structure. The investigation is restricted to boron concentration profiles which are uniform throughout the i-layer, or which decay exponentially with distance from the p-layer. Modest increases in open-circuit voltage result.
Despite the potential utility of these dopant profiling techniques in producing a more favorable field distribution, none has yet addressed the problem of suppressing back diffusion near the front contacting layer. Moreover, all dopant profiling techniques suffer a common limitation: they can, at best, redistribute the built-in electrostatic potential difference, which is bounded in turn by the difference in work function between the two ends of the device. Hence, increases in short-circuit current are accompanied, at most, by secondorder increases in open-circuit voltage.